Techniques of preparing polymer dispersed liquid crystal (PDLC) shutters or light modulating materials by phase separation, as well as the advantages offered by such techniques and the materials prepared thereby, are discussed in U.S. Pat. Nos., 4,671,618, 4,673,255, 4,685,771, and 4,688,900, the disclosures of which are hereby incorporated by reference.
PDLC materials are prepared by forming a homogenous solution of liquid crystal and matrix providing material, and thereafter phase separating so as to cause spontaneous formation of liquid crystal microdroplets dispersed in a light transmissive synthetic resin matrix. The size of the phase separated microdroplets is about that of the wavelength of light.
Electrically addressable, light modulating materials prepared by phase separation techniques and employing liquid crystals exhibiting positive dielectric anisotropy are opaque to incident light in the absence of an applied electric field (OFF-state) and are transmissive to incident light in the presence of a field (ON-state).
The operation of electrically addressable PDLC light modulating materials is due to the ability to manipulate the orientation of the liquid crystal within the microdroplets. In the OFF-state, the directors of the various microdroplets are randomly aligned within the matrix causing an overall mismatch in the indices of refraction of the birefringent liquid crystal with concomitant scattering of incident light. In the ON-state, the electrical field causes the microdroplet directors to align with the field, thereby "aligning" the ordinary index of refraction of the liquid crystal with of the matrix. Since the liquid crystal's ordinary index matches that of the matrix, incident light is transmitted.
The response time of such devices, that is, the time required for all the microdroplet directors to be switched by the applied field from random to ordered alignment (opaque to transmissive switch) and then return from ordered to random alignment (transmissive to opaque switch) depends upon how rapidly the swarms of nematic liquid crystal can respond to the external stimulus of the electric field and then relax when the field is removed.
For day-to-day applications of electrically addressable liquid crystal displays (LCD), such as television screens, alphanumeric displays as in calculators, watches and the like, all of which are operated within a fairly narrow room temperature range usually extending no more than ten degrees from about 20.degree. C. to about 30.degree. C., response time has not been a serious problem.
Heretofore, however, LCD's of the twist nematic kind, as are well known in the art, have been rendered inoperable at low temperatures due to unworkable switching times between the transmissive and opaque states. The operational mode of twist-type LCD's is such that at lowered temperatures, the increased viscosity of the liquid crystal effectively inhibit operation of the twist cell.
It has been discovered that certain liquid crystal light modulating materials fabricated by phase separation techniques surprisingly function at extended temperature ranges reaching downward to about -40.degree. C. and are capable of responding quickly and efficiently at these low temperatures.